Transluminal angioplasty involves the nonsurgical widening of a passage through an artery that has been narrowed, or stenosed, by deposits of plaque or plaque-ridden tissue. One approach to transluminal angioplasty involves the use of laser energy to vaporize, ablate or otherwise remove the plaque deposits. A catheter containing one or more optical fibers is advanced through an artery until its distal end is positioned adjacent to an obstruction. Laser energy sufficient to ablate the obstruction is directed through the optical fibers. A catheter for removal of biological obstructions with laser energy is disclosed in U.S. Pat. No. 4,817,601, issued Apr. 4, 1989 to Roth et al. A laser catheter including multiple optical fibers and adapted to be guided to a target site by a guidewire is disclosed in U.S. Pat. No. 4,850,351, issued Jul. 25, 1989 to Herman et al. Laser catheter systems utilizing infrared wavelengths for tissue issued Apr. 17, 1990 to Sinofsky, and U.S. Pat. No. 4,950,266, issued Aug. 21, 1990 to Sinofsky. Wavelengths in the visible and ultraviolet wavelength ranges have also been utilized for removal of biological material.
When laser energy is used to vaporize or ablate atherosclerotic plaque, thermal damage to surrounding normal tissue is a serious risk. The diameter of arteries is on the order of one to a few millimeters, and the energy level used for ablation of plaque is sufficient to damage or destroy normal tissue. Due to the frequent bends in arteries, the distal end of a laser catheter may be directed at an artery wall rather than at obstructing material. Inadvertent perforation of an artery with laser energy can have serious consequences. The use of pulsed laser energy for tissue ablation, with pulse parameters selected to minimize the risk of thermal damage, is disclosed in U.S. Pat. No. 4,800,876 issued Jan. 31, 1989 to Fox et al.
Since the ablation procedure is performed with a catheter, the tissue being ablated cannot be directly observed. Endoscopic techniques are usually impractical, since the fiber optics for illumination and viewing would increase the diameter of the catheter and reduce its flexibility to an unacceptable degree. Fluoroscopic techniques permit the location of the catheter to be determined, but do not identify the type of tissue being removed.
Techniques have been proposed for distinguishing between plaque and normal tissue by stimulating fluorescence from tissue in an artery and analyzing the frequency spectrum of the fluorescence. U.S. Pat. No. 4,785,806, issued Nov. 22, 1988 to Deckelbaum, discloses the use of ultraviolet laser energy for stimulating fluorescence. Fluorescence intensity at selected wavelengths in the blue/green wavelength range is analyzed to distinguish between plaque and normal tissue. The use of a dye to enhance the contrast between the fluorescence from plaque and the fluorescence from normal tissue is disclosed in U.S. Pat. No. 4,641,650, issued Feb. 10, 1987 to Mok. The use of visible light to stimulate fluorescence from atherosclerotic plaque is disclosed in U.S. Pat. No. 4,718,417, issued Jan. 12, 1988 to Kittrell et al.
It has been recognized that atherosclerotic plaque may have different characteristics, depending on a variety of factors. In particular, calcified plaque is relatively dense and requires more energy for ablation, whereas noncalcified plaque is softer and is more easily removed. G. Laufer et al in Lasers in Surgery and Medicine, Vol. 9, 1989, pages 556-571, describe application of 308 nanometer excimer laser energy to various types of arterial tissue, including normal arterial tissue, fibrous plaque, lipid plaque and calcified plaque. The spectral emission from the tissue was studied. Laufer et al state that calcified plaque exhibits a spectral line shape that is quite different from that of uncalcified arterial tissue. The feasibility of spectroscopic target tissue characterization and a real time spectroscopically-guided device are suggested.
R. H. Clark et al in Lasers in Surgery and Medicine, Vol. 8, 1988, pages 45-59, describe research on laser spectroscopic measurements of cardiovascular tissue including laser Raman light scattering, laser-induced plasma photoemission, laser-induced fluorescence and photoinduced electron paramagnetic resonance. The photoemission from calcified coronary arteries is discussed. The authors suggest that the measurements can serve as monitors during the course of laser photoablation.
M. R. Prince et al in IEEE Journal of Quantum Electronics, Vol. QE-23, No. 10, October 1987, pages 1783-1786, describe the differences between laser ablation of calcified plaque and noncalcified plaque. The authors believe that a laser induced plasma creates a shock wave that assists in ablation of the calcified plaque.
P. Teng et al in Appl. Phys. B, Vol. 42, pages 73-78, 1987 describe studies of laser fragmentation of biliary calculi. The temporal and spectral characteristics of the flash of light accompanying fragmentation of gallstones were studied. Formation of a plasma is suggested.
It is desirable to provide an angioplasty system which is capable of accurately identifying and ablating both calcified and noncalcified atherosclerotic plaque. The system should minimize the risk of damage or perforation of normal arterial tissue.
It is a general object of the present invention to provide improved methods and apparatus for angioplasty.
It is another object of the present invention to provide methods and apparatus for identifying and ablating calcified and noncalcified atherosclerotic plaque.
It is a further object of the present invention to provide methods and apparatus for determining the presence of calcified plaque at a target site in a blood vessel by analyzing a time domain signal representative of calcium photoemission from tissue at the target site.
It is a further object of the present invention to provide methods and apparatus for identification of atherosclerotic plaque at a target site in a blood vessel utilizing analysis of fluorescence by noncalcified plaque and analysis of calcium photoemission by calcified plaque.
It is yet another object of the present invention to provide methods and apparatus for distinguishing between photoemission from calcified plaque and photoemission from a damaged optical fiber.